The present invention relates to tone receivers for communication systems, and more particularly, to multifrequency tone receivers especially adapted for use in pulse code modulation (PCM) communication systems.
In prior art configurations, tone receivers have typically been of the analog type since both speech signals and supervisory tone signals have been transmitted over communication channels in analog form. Because digital techniques provide very high quality signal transmission which is not degraded over long distances, communication systems have recently adopted digital transmission techniques such as PCM. Accordingly, since both speech signals and supervisory tone signals are digitally encoded in such communication systems, a digital tone receiver is preferable over a conventional analog tone receiver.
There are, of course, digital tone receivers known in the art of one type or another. One such receiver apparatus is set forth and described in U.S. Pat. No. 4,354,248, Conger et al., assigned to the same assignee as the present invention. As therein disclosed, a microprogrammed sequence controller is utilized with a time multiplex digital filter and a signal processing microcomputer for filtering digital signals. By controlling input signal samples off the PCM data bus through the time multiplex digital filter, the controller causes the samples to be processed by the microcomputer in a pipeline-like fashion.
A typical application of the above referenced receiver apparatus is the detection of multifrequency tone signals such as those utilized for supervisory signalling in communication systems. In telephony applications, supervisory signals include dial tone, busy, off-hook, etc. These signals are comprised of two frequencies, requiring that the receiver apparatus filter two frequencies at the PCM channel carrying the signal. By sequentially extracting digital samples representative of those frequencies from the input signal, the samples are sequenced through specific filter operations located within the time multiplexed digital filter. The sequential manner in which the digital samples are filtered limits the number of PCM channels that can be handled. This is due to the pipeline-like architecture of this receiver apparatus.
Additionally, communication systems often require applications using multiple signalling schemes. Signalling schemes, such as DTMF or MF, are required for telephony reception. One application may be to configure the system for customer phone usage. In this instance, DTMF may be the required signalling scheme. At a particular time of day, an application may require that tests be executed for remote peripherals, such as a base station transmitter. Often this type of periodic testing demands an alternate signalling scheme, or a special test tone. In a cellular communication system, it is often desirable to remotely test a mobile telephone. This is typically done by transmitting a special test tone from the mobile to a receiver apparatus in the communication system. The special test tone is selected such that it will not interfere with other frequencies from the standard signalling schemes. Although the test tone is used very infrequently, prior art configurations require that a separate receiver apparatus be used for filtering the special test tone.
Typically, digital receiver circuits are arranged in parallel, each designated to a particular signalling scheme. Each circuit independently accesses a PCM data bus with each receiver assigned to each PCM channel. The channels are specified by hard wired addressed lines connected to each circuit. In order to change from one signalling scheme to another, the active receiver circuit must be disabled. This is usually done by toggling an enable signal, wired along the PCM bus, to each card. Conversely, the other receiver circuit is then enabled by toggling its enable signal. This can be very costly as well as potentially unreliable. The cost is incurred by the redundant circuitry of the inactive receiver circuits. The reliablilty loss occurs when one receiver circuit experiences a fault and loads the PCM data bus. For a system with a large number of such receiver circuits, it is extremely difficult to analyze which receiver circuit is actually causing the difficulty. If the difficulty delays reception, the entire system experiences down-time.
This reliability problem has been addressed in prior art by employing a back-up set of receiver ciruits. Each set defines a subsystem. This allows the communication system to redundantly switch from one such subsystem to another when a fault is detected. A major disadvantage of this type of configuration is, again, the cost of the redundant circuitry. This extra cost includes periodic testing and maintenance of the redundant equipment.
A circuit arrangement is needed to overcome the problem of reliability without introducing excessive costs. Additionally needed is a receiver arrangement that can accommodate a variety of signalling schemes, dependent upon system requirements.